1. Field of the Invention
The present invention relates to an image display apparatus, and particularly to a technique for determining a center of rotation for rotating a three-dimensional image representing an observation target to display a rotated image.
2. Description of the Related Art
An advance of image processing techniques in recent years made it possible to generate projection images by projecting a three-dimensional image from various angles by using various image generation methods, such as a volume rendering method and a surface rendering method. Further, the generated images are displayed on a display screen, as images in which an observation target in the three-dimensional image has been moved, enlarged, rotated or the like, by a user's operation on the display screen by a mouse or the like, and they assist the user in three-dimensionally recognizing the observation target.
Conventionally, when an observation target in such a three-dimensional image is rotated at various angles and displayed, it has been necessary to determine a center of rotation in the observation target in advance, or to determine centers of rotation of the observation target in plural projection images, which were projected in at least two different directions, by user's manual operations.
Japanese Unexamined Patent Publication No. 2009-011564 (Patent Document 1) discloses a method for image-based diagnosis in medical fields. In the method, when a region to be displayed is selected, centers of gravity are calculated, as centers of rotation, for all parts in the selected region, respectively. Further, a rotation axis and a viewpoint are determined with reference to a table in which the direction of a rotation axis and the direction of a view line have been set in advance for each anatomical region. Consequently, rotated display is performed appropriately for a region.
However, in the method disclosed in Patent Document 1, or in the method in which centers of rotation are determined by user's manual operations in two-dimensional images reconstructed from at least two different angles, it is impossible to omit manual work for setting a rotation axis and a viewpoint in advance to determine a center of rotation for displaying a rotated anatomical region on a display screen.
Further, in the method in which a center of rotation of an observation target is determined in advance, it is impossible to satisfy a demand of a user who wants to display an observation target in such a manner that a specific region of interest in the observation target is a center.